1. Field of the Invention
The present invention relates to a power steering apparatus and, more particularly, to an improved power steering apparatus wherein the pressure fluid supplied to a hydraulic actuator is controlled in response to the cruising speed of the vehicle.
2. Description of the Prior Art
In a recently developed power steering apparatus, a by-pass passage connecting both chambers of the hydraulic actuator is opened to decrease steering gain at the high cruising speed of the vehicle so that high cruising speed can be safely achieved. If the opening and closing of the by-pass passage is controlled by a solenoid valve, the temperature thereof is elevated when the same is operated for an extended time. Therefore, the resistance of the coil or coils of the solenoid valve is increased. Accordingly, when the high speed drive is restarted during a long distance drive after a temporary stop, an attractive force strong enough for proper operation is not effected because the resistance of the coil has been increased with the result that not enough current can be passed therethrough.
It should be noted that is is quite difficult to use a large capacity solenoid valve because of the undesirable power consumption increase and because the space for the solenoid valve is limited.
Furthermore, at the start of operation, a strong force is required because the magnetic flux is weakened by the space between a movable iron core and a stationary iron core of the solenoid. However, after the spool of the solenoid valve has been displaced to the retraction end thereof, the spool is maintained in this position by a small force.